1. Field of the Invention
The present invention relates to an optical recording medium having at least a dye-containing recording layer and a reflective layer formed on a transparent substrate. More particularly, the present invention relates to an optical recording medium which satisfies the Orange Book Standards for compact disc-recordable (hereinafter, referred to as CD-R), allowing recording and reproduction of information by using a laser beam having a selected wavelength of from to 830 nm as well as allowing reproduction by using a laser beam having a selected wavelength of from 630 to 690 nm.
2. Description of the Related Art
Conventional single-substrate recordable optical disc, CD-Rs, are disclosed in, for example, "Optical Data Storage 1989", Technical Digest Series, Vol. 1, 45 (1989), in which the medium has a recording layer of dyes, a metal reflective layer provided on the recording layer to improve the reflectance, and a protective layer provided on the latter. The recording layer of the media may contain cyanine dyes or phthalocyanine dyes of the type used in the present invention. Such recording media are available on the market as a compact disc-recordable (CD-R).
The CD-R medium is based on the Orange Book Standards and has a reflectance of 65% or higher to a beam having a selected wavelength (.lambda.1) of from 770 to 830 nm while absorbing the beam having the wavelength .lambda.1. Information can thus be recorded on such a medium by using a semiconductor laser of 780 nm. Conveniently, the recorded information can be reproduced through a commercially available CD player or CD-ROM player provided with a semiconductor laser of 780 nm.
On the other hand, optical recording media currently available have a recording capacity of only 650 MB. A recording time is thus 15 minutes at maximum for recording a large volume of information such as dynamic digital images. In the recent trend of downsizing equipment, conventional recording density cannot provide a sufficient memory capacity as the medium becomes small.
Quite recently, a semiconductor laser having a wavelength (.lambda.2) selected from 630 to 690 nm has been developed. This semiconductor laser is making it possible to achieve high-density recording and/or reproduction. It has been studied to develop, with this semiconductor laser, a high-density recording medium having 5-8 times the recording capacity of conventional media and to develop a CD player for such high-density recording media which is capable of reproducing information thereon. In particular, there have been studied systems capable of recording a movie of 2 hours or more in digital format. Read-only media and reproduction players for such systems are to be introduced into the market as digital video discs (DVDs).
As one of this high-density recording media, there is a read-only medium having an aluminum reflective layer deposited on the pits formed in producing the substrate, as in the conventional CD and CD-ROM media. This read-only high-density recording medium has a reflectance of 70% or higher. It is thus necessary that a player capable of reproducing such high-density recording media is designed to have a capability of reproducing information on the read-only medium having the reflectance of 70% or higher. As a matter of course, this high-density player is desired to be able to read conventional media such as CDs, CD-ROMs, and CD-Rs.
Conventional CD and CD-ROM media are for read-only purpose, and the process of manufacturing them is the same as that for the above mentioned high-density read-only media. These media have a reflectance of 70% or higher and are thus applicable to the high-density player for reproduction.
On the other hand, the conventional CD-R media currently available in the market have a reflectance of 65% or higher to the light beam having a wavelength of about 780 nm and are applicable to a commercially available CD or CD-ROM player. However, these media have a reflectance of 10% or smaller when subjected to reproduction with a light beam having a wavelength (.lambda.2) selected from the range of 630-690 nm. The degree of modulation is small and the reflectance of a recorded portion becomes larger than that of an unrecorded portion. This phenomenon is known as "low to high recording", in which the polarity is reversed from common CDs (high to low recording). In addition, a resultant reproduced signals have a large deformation. Due to these various disadvantages, it is difficult to play the conventional CD-R media by using a player for high-density media with a laser beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm.
For example, U.S. Pat. No. 5,090,009 discloses a CD-R medium comprising a dye-containing recording layer, a reflective layer, and a protective layer laminated on a substrate in this order. This patent also discloses a medium having an interference layer provided between the substrate and the recording layer or between the recording layer and the reflective layer. In addition, it discloses optical constants as well as a film thickness of the recording layer to satisfy the CD standards (Red Book) and to enable recording. The media disclosed certainly absorb a portion of the light beam of 780 nm which is used for CDs. In addition, the reflectance of the media can be 70% or higher. The CDs are thus recordable and reproducible with the light beam of this wavelength. The '009 patent is, however, not directed to recording and reproduction with the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. In addition, there is no disclosure about the reflectance to the light beam having that wavelength, the optical constants of the recording layer, and an optical path length of the interference layer. The '009 patent discloses various examples of the media having a recording layer of an indodicarbocyanine dye, of which some media have an interference layer made of an inorganic compound or a polymer, and others not. However, the media with the recording layer of the indodicarbocyanine dye have the reflectance of 10% or lower to the light beam having the wavelength selected from the range between 630 and 690 nm. The recording is the low to high recording, in which the reflectance of the recorded portion is larger than that of the unrecorded portion. The reasons of this small reflectance are as follows: the optical characteristics of the dye used is highly dependent on the wavelengths. The above mentioned indodicarbocyanine dye has a significantly large absorption at from 600 to 750 nm when measured on the recording layer; and the CD-R medium has the optical constants (refractive index which is the value of the real part of the complex refractive index, and attenuation coefficient which is the absolute value of the imaginary part of the complex refractive index) and the film thickness designed to achieve a high reflectance to the light beam of around 780 nm. This means that the reflectance is small at the wavelengths within the range between 630 and 690 nm. In addition, the Examples 8 and 14 of this patent disclose media having a recording layer of t-butyl substituted phthalocyanine dye, which is different from phthalocyanine dye used as the recording layer of the present invention, and an interference layer made of a polymer or an inorganic compound. Though the reflectance and the degree of modulation of these media satisfy the CD standards, a reproduced waveform has a large deformation when the signal recorded by pulse width modulation as in CDs is reproduced with the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. The error rate and the jitter value become large, and the media thus cannot be played back by a player for the high-density.
European Patent Laid-Open No. 0 019 329 discloses a medium having a recording layer of VO-phthalocyanine dye, on which an interference layer of cellulose (300 nm) is applied. The application is, however, directed to improve the reflectance and the recording sensitivity to a specific single wavelength. The recording layer is not optimized to permit recording and reproduction with two light beams having the wavelength (.lambda.1) selected from the range between 770 and 830 nm and the wavelength (.lambda.2) selected from the range between 630 and 690 nm, respectively. For this medium, a reproduced waveform has a large deformation when the signal recorded by pulse width modulation as in CDs is reproduced with the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. The error rate and the jitter value become large, and the media thus cannot be played back by a player for the high-density.
U.S. Pat. No. 5,124,067 discloses dyes similar to those used in the recording layer of the present invention. The patent also discloses several media using such dyes. Though satisfying the CD standards, the media disclosed in Examples 98, 100, and 102 of the patent provide a recording mode of low-to-high when reproduced with the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. In addition, the reflectance is 10% or lower. Furthermore, a reproduced waveform has a large deformation when the signal recorded by pulse width modulation as in CDs is reproduced with the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. The error rate and the jitter value become large, and the media thus cannot be played back by a player for the high-density.
Japanese Patent Laid-open No. 3-281287 discloses a medium having a recording layer of a mixture of dyes (A) and (B), in which the dye (A) has an absorption maximum of the longer wavelength at 780 nm and has such a complex refractive index that the absolute value of the imaginary part of the refractive index at 780 nm is 0.2 or smaller; and the dye (B) is a monocarbocyanine dye having an absorption maximum at a wavelength shorter than the wavelength at which the dye (A) exhibits the absorption maximum. This medium is superior in durability and satisfies the CD standards. However, it is not optimized for reproduction with the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. As apparent from the examples, the rate of the monocarbocyanine dye used is 50 wt.% and the rate of the dicarbocyanine dye used is also 50 wt. %. The resultant medium thus has an excessively large absorption to the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. As a result, the reflectance becomes lower than 15%, and it is thus difficult to reproduce the recorded information by using a player for the high-density.
Japanese Patent Laid-open No. 6-336086 discloses a medium having a recording layer of a mixture of a monocarbocyanine dye and a dicarbocyanine dye both of which have a specific structure. This medium is directed to recording and reproduction with the light beams having the wavelength of 780 nm and 488 nm. A ratio of the monocarbocyanine dye and the dicarbocyanine dye is 1:10. With the larger rate of the pentamethine cyanine dye used, the resultant recording layer has an excessively larger absorption to the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm. As a result, the reflectance becomes lower than 15%, and it is thus difficult to reproduce the recorded information by using a player for the high-density.
Japanese Patent Laid-open No. 6-40162 discloses a medium having a recording layer of a monocarbocyanine dye. This medium is directed to recording and reproduction with the light beam having the wavelength of 630 nm. The recording layer has no absorption to the light beam having the wavelength of 780 nm, so that the recording cannot be achieved With the light of this wavelength.
Japanese Patent Laid-open No. 3-290835 discloses a medium provided with an interference layer made of a low molecular weight organic compound between a recording layer and a reflective layer of an aluminum alloy. This medium uses the aluminum alloy rather than expensive gold for the reflective layer, and has an interference layer to achieve the reflectance of 70% or higher at 780nm. In this medium, the reflectance is certainly 70% or higher at 780 nm. However, the reflectance is low to the light beam having the wavelength (.lambda.2) selected from the range between 630 and 690 nm, and it is thus difficult to reproduce the recorded information by using a player for the high-density.
As mentioned above, there is no such an optical recording medium having at least a dye-containing recording layer and a reflective layer as has a reflectance of 65% or higher to the laser beam having the wavelength (.lambda.1) selected from the range between 770 and 830 nm, has high sensitivity, is superior in recording characteristics, can be played back by a commercially available CD or CD-ROM player, and as can be played back also by a player for high-density with a laser beam having the selected wavelength (.lambda.2) of from 630 to 690 nm.